The present embodiments relate to medical diagnostic ultrasound imaging. In particular, pressure control of the transducer against the patient is provided.
In ultrasound imaging, the user places a transducer against the patient. For good acoustic coupling, the user presses the transducer towards the patient. If there is too little compression, the acoustic coupling may be insufficient. Too much pressure may cause patient or sonographer discomfort and may adversely distort tissue. The amount of contact force to apply is a subject of user training, requiring numerous scans to achieve proficiency.
Image artifacts may result from incorrect pressure. For example, a patient's breast is to be scanned in three dimensions with ultrasound using a scanning pod. Over-compression of the breast by the scanning pod or transducer may lead to two related compression artifacts. The first artifact is a reduction in anatomic resolution caused by compression of small tissue structures. The physical separation and/or impedance contrast between adjacent structures cannot be resolved in the ultrasound image if the pressure causes the tissue boundaries to be very close together. This loss of information results in a loss of diagnostic yield and a loss of confidence for detecting small pre-cancerous or neo-cancerous lesion detection.
The second artifact is a reduction in anatomic resolution due to blurring of tissue over small regions. The transducer may be moved to scan different planes for three-dimensional imaging. As the transducer moves laterally, friction between the transducer face and the tissue causes lateral displacement of the tissue and an increase in static shear force. When this shear force reaches a certain threshold—defined by properties including the contact normal force, tissue stiffness, tissue shape and contact lubrication—the contact begins to slip, leading to a relaxation of the tissue counter to the transducer movement. Following some relaxation of the tissue, the contact sticks again and the process repeats. This is referred to as “stick-slip.” The result within an image of a volume is a blurring of image detail corresponding to locations scanned during tissue relaxation.